The iron isotope composition of microbial carbonate

Abstract

In a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve Fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation. It was found that δ 56Fe values of Archean stromatolites vary between − 2.1‰ and − 0.5‰, Devonian and Carboniferous microbiallites vary between 0‰ and − 1‰, and modern microbialites from the Great Barrier Reef show a range from − 0.12‰ to + 0.15‰. Five lines of evidence support the possibility that these compositions are potentially pristine. 1) Fe concentrations and Fe isotope ratios are not correlated. 2) The concentrations of other elements that are potentially mobile during carbonate diagenesis (Mg, Sr) do not correlate with δ 56Fe. 3) Mn concentrations are inversely correlated with δ 56Fe, which might hint at meteoric alteration. However, if Mn concentrations and Fe isotope signatures were pristine features, they might serve as a record of the reduction of these two metals in seawater. 4) Dolomitisation of two Devonian limestones does not shift their Fe isotope compositions. 5) Rare earth element and yttrium (REY) patterns in all limestones are similar to seawater REY sources. This suggests that the bulk of the Fe in the samples occurs in hydrogenous carbonates, not in primary or secondary Fe and Mn hydroxides, because the latter have different REY patterns. The modern microbialites are unfractionated in Fe relative to crustal rocks. Thus, these hydrogenous precipitates either incorporated unfractionated Fe of the ambient seawater composition, or they modified the Fe composition upon incorporation from a fractionated ocean Fe reservoir. In either case, provided diagenetic alteration has not modified the Fe isotope composition in a major way, a potential open ocean Fe isotope proxy may exist that may allow reconstruction of secular variations in seawater Fe isotope compositions. Our preliminary results suggest that Neoarchean microbial carbonate Fe may have been lighter than today's microbialite Fe, and that the carbonates' Fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time.